Why?

Why smile high stars the happier after rain?
Why is strong love the stronger after pain?
Ai me! ai me! thou wotest not nor I!
Why sings the wild swan heavenliest when it dies?
Why spake the dumb lips sweetest that we prize
For maddening memories? O why! O why!
Why are dead kisses dearer when they're dead?
Why are dead faces lovelier vanished?
And why this heart-ache? None can answer why!